probe: Add multi_probe_begin() / multi_probe_end() callbacks

Add support for notifying the low-level probing handlers that a
multiple probe sequence is occurring.  This is in preparation for the
bltouch code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/bltouch.py

@@ -137,6 +137,10 @@ class BLTouchEndstopWrapper:
         # Test was successful
         self.next_test_time = check_end_time + TEST_TIME
         self.sync_print_time()
+    def multi_probe_begin(self):
+        pass
+    def multi_probe_end(self):
+        pass
     def probe_prepare(self):
         self.test_sensor()
         self.sync_print_time()